The invention relates to a method of coating a luminescent material with a layer of a metal oxide M.sub.2 O.sub.3 in which a metal M is chosen from the group formed by Y and Al. The invention also relates to a coated luminescent material and to a low-pressure mercury discharge lamp.
A method as mentioned in the opening paragraph is known from European Patent EP 0 488 330. Small particles of the metal oxide and the luminescent material are brought together in an aqueous environment in the known method. After homogenization through stirring, the solid substance is separated from the liquid and dried. In the known method, the metal oxide/luminescent material ratio is so chosen that the metal oxide covers only a comparatively small portion of the surface area of the luminescent material. The use of the luminescent material obtained by the known method in a low-pressure mercury discharge lamp led to an improvement of the lumen maintenance of the low-pressure mercury discharge lamp compared with that of lamps in which uncoated luminescent materials are used. This improved maintenance is probably due to a less strong adhesion of HgO to the surface of a luminescent material coated with metal oxide. Besides adsorption of HgO at the surface of the luminescent material, adsorption and absorption of Hg also occurs in a low-pressure mercury discharge lamp. This absorption of Hg has a number of major disadvantages. First, Hg disappears from the plasma as the low-pressure mercury discharge lamp ages. This decrease in the quantity of Hg in the lamp plasma renders it necessary to introduce a comparatively large quantity of Hg into the lamp in the manufacture of the low-pressure mercury discharge lamp. The luminous efficacy of the low-pressure discharge lamp, however, is comparatively low with this comparatively great quantity of mercury. Other disadvantages of the mercury absorption are the fact that the optical properties of the luminescent material are changed thereby, while the presence of Hg also adversely affects the recycling possibilities of the luminescent material.